1. Field of the Invention
The invention relates to apparatus for the regeneration of channel-clock information from a series of samples which represent the signal values of a band-limited transmission signal at equidistant sampling instants which transmission signal is synchronous with the channel clock, comprising computing means for deriving first positions of sampling instants relative to reference instants defined by fixed phase positions of a reference clock from the positions determined for previous sampling instants, detection means for detecting crossings of a detection level by the transmission signal between successive sampling instants from the values of samples corresponding to said sampling instants, interpolation means which are responsive to a level-crossing detection for determining second positions of sampling instants relative to the level crossings from the values of samples in the proximity of the detection-level crossings, and means for comparing the second positions and the first positions of sampling instants with each other.
The invention also relates to a circuit arrangement for the recovery of data represented by the sampled band-limited transmission signal, comprising apparatus for the regeneration of channel-clock information.
2. Description of the Related Art
Such apparatus and such a circuit arrangement are known from European Patent Application No. EP 0,109,837. The known channel-clock information regeneration apparatus is employed for the recovery of digital data represented by a band-limited binary synchronous data signal obtained from a magnetic-tape reader. In this apparatus the data signal is sampled at a rate which is approximately twice the channel-clock frequency. For each sample the channel-clock information regeneration apparatus determines the phase of a virtual reference clock signal as a measure of the position of the sampling instant. For each sample the measure of the position is adapted by a value corresponding to 180.degree. by means of a cyclic counter whose counting range corresponds to a 360.degree. phase difference. The count of the cyclic counter represents the phase of the virtual reference clock. Each time that the count exceeds a value corresponding to a phase position of 0.degree. between two consecutive samples the data recovery circuit arrangement detects one bit of a sign which is dictated by the sign of the last sample.
Further, in response to a detection-level crossing, the phase difference between a sampling instant adjacent the detection-level crossing and the detection-level crossing is derived from the sampling values of the samples on opposite sides of the detection-level crossing. The phase difference between the channel clock and the virtual reference clock is derived from the count of the counter and the phase distance from the level crossing thus determined. The count is adapted by a value which depends on this difference. In this way it is achieved that the phase represented by the count of the cyclic counter substantially corresponds to the actual phase of the channel clock at the sampling instants.
However, such known apparatus for the regeneration of the channel clock information has the disadvantage that for reliable operation it is necessary that the sampling rate be substantially equal to twice the channel-clock frequency. If the sampling rate deviates from this value the phase difference between two successive samples no longer corresponds to 180.degree., so that the phase represented by the count of the cyclic counter deviates increasingly from the actual phase of the channel clock as the time interval between the sampling instants and the last detection-level crossing increases.